I always abide by common sense when overclocking a graphics card. Generally I start at about a 10% increase and monitor the PC for any errors, over heating or BSODs for a day or two, I personally wouldn't go above 15% with out some extra cooling, mainly because I don't like monitoring the temperatures the whole time.

This is just my opinion and how I do things, it may not necaserily be the best, safest, or most beneficial way of doing it.

Oli

Hello Prak06,
Rive Tuner is your best bet here, (link below), but to be honest you wont see that much of an improvement in FPS, all it will do is shorten the life of the card, and of course raise the temperature, those 8800's can get quite toasty as it is.
If you go ahead with this, go in small steps, as osholt points out, 10% is about it.

RivaTuner v2.24c download from Guru3D.com

Hope this helps, good luck and post your results.

I'd use EVGA Precision or MSI Afterburner.
They are easier to work with if your new to the 3rd party OC utility.

As for settings, Your 8800 GT is running unusually hot for idle (unless that 60c is ingame). So keep it at 100% if it is idle.
I had an 8800 GT SC at 700 mhz core clock (most i could go), 1650 shader, and for the memory somewhere near 975 (take 20-25 mhz increments).

I'd recommend trying MSI Afterburner first as it comes with a built in core voltage setting that lets you have a more stable overclock.

EVGA_Precision_Setup_194.exe

MSIAfterburnerSetup160.zip